This invention relates to those tools known as screw taps, used to create or regenerate a female thread. In particular, the invention relates to those tapping devices used to regenerate female threads located in positions in which any fall of cutting swarf is undesirable.
Tapping tools of known type generally consist of a screw portion interrupted by cuts or grooves for escape of the swarf. Said grooves, which are equidistant, are directed along the screw axis, or extend helically along the outer surface of the screw.
Although the known devices perfectly satisfy the purpose for which they are proposed, under certain operating conditions they present drawbacks. For example, while regenerating the thread of the seat for a spark plug, it can happen that the swarf falls into the combustion chamber, from which it must be removed to prevent damage to the engine. The same happens when regenerating for example the Lambda probe of catalytic exhaust systems. Again in this case considerable care must be taken, because even a single shaving can irremediably damage the catalyst.
Currently, when the operator has to carry out one of the described operations he is compelled to use expedients, such as to spread solid lubricant on the surface of the screw tap in the hope that the swarf remains attached to it. Another expedient used is to insert rags into the hole to be threaded or rethreaded.
The object of the present invention is to overcome the drawbacks of the known art, within the context of a rational and reliable solution which dispenses with the need for the use of any expedient by the operator.
The invention attains said object by means of a tapping tool comprising a threaded portion and an upper gripping portion or shank, and is provided with an axial dead bore which has its base wall proximate to the lower end of said threaded portion and opens to the outside at the upper surface of said shank. In said threaded portion there is present at least one duct arranged to connect said axial dead bore to the threaded portion, the axis of said at least one duct being inclined to the radius of said screw portion, both to enable a sharp edge to be formed on one side of the rim of said at least one duct, and to enable the swarf to roll into the dead bore of the tool.
At the lower end of said threaded portion there is present a cylindrical nosepiece which enables the tool to be easily centered within the hole to be threaded and consequently to execute correct rethreading (or tapping) perpendicular to the hole.
The tool of the invention can be provided with a ring nut to act as a reference element for the depth of the thread. A locking nut is preferably associated with said ring nut.
Said ring nut also preferably presents a flat surface on one side, and on the other side an inclined surface which matches the sealing surface present on the spark plug intended for the hole being threaded.
In a variant of the invention, the cavity of said gripping shank is threaded and constitutes a screw die for the outer surface of the shank for example of the spark plug.
Obviously in this case the sharp edge consists of at least one inclined hole arranged to expel the cutting swarf to the outside.
The constructional and operational characteristics of the invention will be more apparent from the ensuing description of a preferred embodiment thereof given by way of non-limiting example and illustrated in the accompanying drawings.